Plasticizers are additives used to increase the plasticity or fluidity of materials. Examples of materials that are commonly plasticized include plastics, clays, and concrete. With respect to polymeric materials such as plastics, it has been hypothesized that plasticizers perform their plasticizing function by embedding between the polymer chains, thus spacing them out and increasing the free volume for the polymer. This increase in the free volume results in a lowering of the glass transition temperature (Tg) for the polymer, thus rendering it more flexible or malleable, in other words, rendering it more plastic.
An important application of plasticizers is for plasticizing polymeric materials, particularly polyvinyl chloride (PVC). By some estimates, over 90% of the plasticizer market is directed to plasticizing PVC. See, David F. Cadogan and Christopher J. Howick “Plasticizers” in Ullmann's Encyclopedia of Industrial Chemistry 2000, Wiley-VCH, Weinheim. Without the addition of a plasticizer, PVC is a rigid material. PVC can be made softer and more flexible by the addition of plasticizers. Plasticized PVC is used as a replacement for rubber, and has applications in many areas including, for example, pneumatic tires, electrical wire and cable insulation, flooring, coatings, tubing, inflatable products, toys, foams, roofing membranes, food packaging, footwear, coats, sporting gear, magnetic cards, hoses, furniture, exterior siding, bottles, inflatable products, examination gloves, flexible sheeting, gaskets, medical devices, containers, and imitation leather.
The most common plasticizers used for plasticizing PVC and other polymers are esters of polycarboxylic acids with linear or branched chain aliphatic alcohols. Examples of these carboxylic acids are phthalic acid and trimellitic acid. Examples of the aliphatic alcohols used to esterify these acids include C6 to C10 alcohols. One of the more widely used plasticizers is diisooctyl phthalate (DIOP or DOP), which is also known by a number of other names including dioctyl phthalate, diethylhexyl phthalate (DEHP), di-2-ethylhexyl phthalate, and bis-2-ethylhexylphthalate.
Despite their utility, plasticizers are not always fully compatible with and leach out of and evaporate from the polymers they are intended to plasticize. For example, it is believed that plasticizers that have leached out of and vaporized from the plastic interior of a car contribute to the characteristic new car smell. The loss of plasticizer from a polymeric material can have negative consequences. Firstly, as the plasticizer is lost from the polymeric material, the surface of the polymer can become sticky or tacky. In some instances the polymeric material releases droplets of plasticizer on its surface. In other words, the polymeric material sweats or weeps. As the plasticizer is continuously lost, the polymeric material can eventually become more rigid and brittle. Thus the desired flexibility characteristics of the polymeric material are lost and the material can fail. Secondly, there are potential health concerns if consumers come in contact with the leached or vaporized plasticizer materials, such as through physical contact or via inhalation or ingestion. Additionally, there are potential environmental concerns due to the leached or vaporized plasticizers being released into the environment. Because of these concerns, there have been movements to limit or ban commonly used plasticizers such as DOP in some countries.
Separate from these safety and environmental concerns with plasticizers, there is an overarching question of sustainability and environmental stewardship in the production and use of products. It would be highly desirable to develop plasticizers that can be prepared from sources other than nonrenewable petrochemical feedstocks. Furthermore, it would be desirable to develop plasticizers that can be prepared by recycling waste streams. The safe disposal or reuse of waste materials from various sources is an environmental and economic challenge. Such wastes had typically gone into landfills, but as landfill capacity is becoming ever scarcer and disposal costs are continuously increasing, cost effective and environmentally acceptable alternatives are needed to deal with these waste materials. For example, a readily available waste stream is produced from waste thermoplastic polyesters, including waste polyethylene terephthalate (PET) streams (e.g., from plastic beverage containers). Therefore, it would be advantageous to find ways to recycle such waste streams into new products.
Regarding the plasticizers themselves, there are the challenges of developing materials having optimal physical and chemical properties. For example, plasticizers that are compatible with and useful for plasticizing PVC should have low acid values, low hydroxyl values, low oxygen ether content, moderate to high molecular weights (in the case of polymeric plasticizers), and viscosities that allow reasonable processing to make plasticized polymeric materials. Plasticizers not meeting the criteria for acid and hydroxyl values can be detrimental to the PVC, causing water uptake of the polymer and more serious issues such as dehydrohalogenation, resulting in acid release, failure of the polymer, and potential damage to materials in contact with the failing polymer. Additionally, the plasticizers should be safe for use in consumer products and not detrimental to the environment. These physical, chemical, safety, and environmental criteria raise difficult technical challenges for developing new plasticizers.
In addition to these forgoing challenges are the technical and economic challenges to cost effectively produce and formulate the plasticizers.
In many instances, it would be highly desirable to have improved plasticizers. It is apparent there is an ongoing need to develop new plasticizers that are compatible with and have the desired technical and performance characteristics for plasticizing polymeric materials such as PVC. It is important that these plasticizers do not easily leach or evaporate from the plasticized polymer and do not have untoward health or environmental concerns. These plasticizers should be technically and economically viable to produce. Furthermore, it would be highly advantageous to develop plasticizers that can be sourced from sustainable sources to employ and reduce waste streams.
We surprisingly found that polymeric plasticizer compositions meeting the foregoing criteria can be made from an aromatic acid source, a glycol, and a component selected from a C4-C36 monocarboxylic acid, or ester or anhydride thereof or further comprising a C4-C36 alcohol.